1. Field of the Invention
This invention relates to apparatus for improving the efficiency of fume hoods by inhibition or elimination of eddy currents from forming in front of an operator.
2. Description of the Related Art
Exhaust hoods are widely used in industry to protect workers from exposure to fumes, aerosols, or any other air borne contaminates to which the worker might otherwise be exposed. Such exhaust hoods may comprise small hoods such as found in chemical labs or may comprise large hoods placed over large pieces of equipment, e.g., a cleaning vat or a boat under construction. The protective structure may even comprise entire rooms or enclosures, such as a paint booth or the like with exhaust ports or vents located either at the rear or top of the enclosure. In each case, however, the exhaust port or vent is usually in a position where the source of contaminants is in between the worker and the exhaust port.
In each such case, the purpose or point of the exhaust means is to remove the airborne contaminates from the enclosure and thereby protect the worker, who is either within or adjacent to the enclosure containing such airborne contaminants. However, George et al. in "The Impact of Boundary Layer Separation on Local Exhaust Design and Worker Exposure", published in Appl. Occup. Hyg. 5(8) at pages 501-509 in August 1990, showed that eddy currents could form in front of a worker due to the positioning of the worker in the air stream flowing toward the exhaust port. In particular, the article showed, in FIG. 1, that air flowing around a circular cylinder formed eddy currents on the downstream side of the cylinder which tend to flow inwardly toward the front or dead space adjacent the downstream side of the cylinder. Thus, when the source of airborne contamination is conventionally positioned between the person and the exhaust port, some of the airborne contamination may be drawn back toward the person by such eddy currents, the exact opposite of the desired effect!
This effect is further illustrated in FIGS. 1 and 2 of the drawings. FIG. 1 shows a top view showing air, from an air source 2 located behind a person identified by numeral 4, flowing around person 4 and the horizontal eddy currents which are developed by this flow of the air around person 4 which tends to draw the pollutants emitted from the source of contamination 6 back toward the person instead of toward exhaust port 8. FIG. 2 is a side view of person 4 of FIG. 1 showing how the air flow from source 2 flowing around person 4 toward exhaust port 8 is further thought to generate vertical eddy currents in front of the person which swirl upwardly in front of the person toward the breathing zone.
Increasing the total air flow into the exhaust hood will not totally remedy this problem. While an increase in the total air flow from a hood face velocity of, for example, about 56 feet per minute (fpm) up to a higher hood face velocity of, for example, about 118 fpm will result in an extension of the eddy current region, resulting in a dilution of the amount of contaminants swept back toward the person by the eddy currents. This results in the need for larger exhaust fans consuming more electrical power, thus adding additional expense to the exhaust system. Furthermore, exhaust hoods with high air flow rates require substantial make-up air to be introduced into a building which must then must be conditioned (i.e., heated or cooled) which takes additional energy. If air pollution abatement regulations require the trapping of pollutants from the exhaust, by filtration of solids and/or condensation of volatiles, operation at high air flow rates will result in yet further added costs.
The aforementioned George et al. article reported that smoke tests conducted with a mannequin indicated that when the mannequin was placed sideways in the airstream, that is, with the air coming and leaving from opposite sides of the mannequin, none of the smoke in front of the mannequin was reported to be drawn toward the breathing zone of the mannequin.
This observation is further reported in another article entitled "Aerodynamics and Exposure Variability" by Flynn and George, two of the three authors of the first publication. In this article, the authors examine the differences in effect of air flow when the air is supplied from the back of the mannequin and the contaminant source is placed in the near wake in front of the mannequin created by the air flow around the mannequin rather than flowing the air toward the side of the mannequin while the contaminant source remains in front of the mannequin.
While these articles appear to conclude that the position of the worker, with respect to both the air source and the contaminant source, can be varied to vary the amount of exposure to contaminants which flow back to the breathing zone of the person due to eddy current generation, it is not always convenient for the worker to vary his/her working position with respect to either the air source and/or the source of the airborne contamination.
What would be desirable is a means for making a person transparent to the air flow, i.e., a means wherein the air flow from the air source would flow toward the contaminant source, and then flow toward the exhaust port (with the contaminant entrained therein), without any obstruction caused by the presence of the person, regardless of his/her location with respect to either the air source or the contaminant source.